I've been playing the Falskaar mod for Skyrim. This starts out with you exploring a Nordic ruin that has a strange looking wall. The wall ends up being a portal to the land of Falskaar. Falskaar is much like Riften in looks and is about the size of the Reach. There are about 20 dungeons and most are more heavily populated than Skyrim's. A good mod with mostly decent voice acting, though the main quest giver is horrible